texts:
This excellent answer to the question What is the physics of the “spinning dust” contribution to Cosmic Microwave Background measurements? includes the figure below which helps show what it was the triggered the existence-hypothesizing of cosmic spinning dust.

The question Why doesn't the equipartition theorem disallow spinning dust? is my way of starting to wonder if spinning dust is real, or if it really happens.

One thing that would make me more comfortable than an argument gedankenexperiment involving a modified Crookes radiometer with the gas removed would be some kind of experiment showing that simply being exposed to some very low pressure hydrogen, or photons, can make dust spin at anything close to 20 GHz!

These days optical trapping of particles in very high vacuum is readily accessible for example, so I think if someone wanted to try to make GHz-spinning-dust they could certainly have a go.

Question: Has this been tried? Has "GHz-spinning dust" ever been demonstrated in the laboratory?


texts:
I have seen a number of places on the Internet where Epsilon Eridani is referred to as Sadira. (e.g. https://en.wikipedia.org/wiki/List_of_stars_in_Eridanus )   However, it would seem that its an unofficial name and the star got renamed Ran by the IAU in a competition.

How did Epsilon Eridani become known as Sadira by some?
What or who was Sadira?


texts: Expression for length of sunrise/sunset as function of latitude and day-of-year

texts: I would like to know how the length of sunrise/sunset (and also time between civil/nautical/astronomical twilight) varies with latitude and with day of year. Ideally what I am looking for is an expression that takes latitude and day-of-year as arguments and outputs the time taken for the sun to rise or set at that latitude on that day.


texts: What other “planetary formation mechanisms” exist beyond “gravitational collapse”?

texts:
My curiosity was pricked by the an edit made to this post: What's the distinction between "planetary formation mechanisms" and "gravitational collapse"? sadly the edit was changing the original questions intend, so I had to roll it back.

But the question of the edit itself felt interesting to me aswell, so I'm reposting it here in another question:

What "planetary formation mechanisms" other than "gravitational collapse" exist?


texts: Calculate Atmospheric Extinction Using Source Altitude Angle

texts:
I am trying to code up the following article http://dl.acm.org/citation.cfm?id=2349512 (alternative free access version) to calculate the light reflected from the moon to a given point on earth at a given time. However I occasionally get a gross error when calculating the distance of the light path through the atmosphere (Figure 2. of the article.)

The approximation given is to take the inverse sine of right ascension in radians, however whenever RA approaches pi, the number blows out to 8e15. Can you suggest a better method?


texts: Would a future Dyson sphere or swarm around our Sun actually be beneficial to people on Earth?

texts:
A Dyson sphere or swarm's function is to harvests most of a star's energy, correct?
However, a future Dyson sphere around our Sun would also block the Sun's rays from reaching Earth. Wouldn't it freeze the people on Earth to death and ruin the bio-sphere?
Or is there some way around that?


texts:
This answer to Are astronomers waiting to see something in an image from a gravitational lens that they've already seen in an adjacent image? describes "Deja vu all over again" (SN "Refsdal"); after multiple gravitationally lensed images of a supernova were observed with different path lengths an shifted light curves, one more "echo" was predicted then indeed was later observed.
While that's a rarity, occultations of objects by other objects are regularly predicted, and some of the nearer objects are pretty far, at least in the Kuiper belt. But alas they are not usually very massive.
Gravitational microlensing events are defined by a peak in a light curve rather than a measurable deflection. From How are microlensing events used to constrain the size of innermost stable circular orbits around spinning black holes?:

Example light curve of Gravitational Microlensing event - OGLE-2005-BLG-006 Source

Question: Has a gravitational microlensing event ever been predicted? If so, has it been observed?

Related to stellar occultations by distant objects:

Where and how to look up the coordinates and magnitude of the star in this occultation prediction?
What is the farthest reported distance from observer from which a solar system body has occulted a star?
What discoveries have come from the DASCH project (Digital Access to a Sky Century @ Harvard)
Could stellar occultations be used to survey Kuiper belt objects?
this answer to Is it possible to mount a telescope on a plane? Is it beneficial?
What caused this mysterious stellar occultation on July 10, 2017 from something ~100 km away from 486958 Arrokoth?
Is 486958 Arrokoth (2014 MU69 aka Ultima Thule) the only solar-system object determined to be binary by occultation?


texts: Is it possible to derive Metallicity/Age/Radial Velocity/Rotational Velocity/Surface Gravity of a star from Hipparcos data files or are do they require individual studying?


texts:
Not all galaxies are spiral in shape[1][2], some nebulae are huge[3] and nebula are the nursery of stars[4]. How to tell them apart?

Note

I have already compare the contents for galaxy and a nebula, they are very similar as both have stars, planets and dusts.


texts: This page on Wikipedia -- Quasars mentions that the "The largest known [quasar] is estimated to consume matter equivalent to 600 Earths per minute". However, there is no citation for this comment. How can I find out where this information came from? I've commented in the Talk section for the page.


texts:
As Wikipedia says the surface temperature of UY Scuti is 3,365 K and the sun is 5,778 K

Why is the Sun hotter than a supergiant star like UY Scuti?


texts: What exactly is the relationship between the orbits of Neptune's moons Naiad and Thalassa?

texts:
Scitechdaily's Wild Orbits of Neptune Moons a ‘Dance of Avoidance’ [Video] links to the YouTube video Neptune Moon Dance (Naiad and Thalassa) and says:

In this perpetual choreography, Naiad swirls around the ice giant every seven hours, while Thalassa, on the outside track, takes seven and a half hours. An observer sitting on Thalassa would see Naiad in an orbit that varies wildly in a zigzag pattern, passing by twice from above and then twice from below. This up, up, down, down pattern repeats every time Naiad gains four laps on Thalassa.
Although the dance may appear odd, it keeps the orbits stable, researchers said.
“We refer to this repeating pattern as a resonance,” said Marina Brozovic, an expert in solar system dynamics at NASA’s Jet Propulsion Laboratory in Pasadena, California, and the lead author of the new paper, which was published Nov. 13 in Icarus. “There are many different types of ‘dances’ that planets, moons, and asteroids can follow, but this one has never been seen before.”
[...]
Reference: “Orbits and resonances of the regular moons of Neptune” by Marina Brozović, Mark R. Showalter, Robert A. Jacobson, Robert S. French, Jack J. Lissauer and Imke de Pater, 22 October 2019, Icarus.
DOI: 10.1016/j.icarus.2019.113462
arXiv: 1910.13612

I think the term "choreography" was meant to be colorful, but not refer to an actual n-body choreography.
Question: Orbital resonances are common enough, but what does "this one" mean in the last sentence "this one has never been seen before”? Some additional questions may help explain what it is that I'm having a difficult time understanding:

What type of orbit does "this one" refer to? Is there a term?
Is this truly resonant behavior?
Are the two periods related by a rational number?
Are there also some significant chaotic aspects to their relative motion?

above: cropped second of a screenshot from the linked video. If I understand correctly this is at least approximately in Thalassa's synodic frame; Naiad slowly moves around Neptune and the planet is shown rotating underneath.
below: more screen shots...


texts: Why doesn’t the Sun fill the sky on Mercury?

texts:
I’ve seen a lot of photos showing Mercury in front of the Sun when it passes by and you can see just how tiny it is in comparison.

Here’s a great example:

So I’d expect if I was travelling towards Mercury in that photo that as the planet got bigger in my view so would the Sun and eventually I wouldn’t be able to see the edges of the sun because it’s so big at this distance... makes sense that an object gets bigger the closer you get to it!

However what confuses me is that if you were on the surface of Mercury... this is the view you’d get where it’s about a 6 times larger than when viewed from Earth.

So how is this so small when it’s taking the whole view up when we’re viewing it further away from mercury in the first photo? But we’re closer in the second photo but now the sun is smaller... How is the first photo possible (it’s an actual photo taken by NASA) why isn’t it taking up the whole sky in the second photo when you’re actually on the planet?


texts:
What is the resolution in megapixels of the Hubble Telescope?

The HST cannot see the planet Pluto clearly. Why is that?


texts: Which are stars and which are noise in this comet photo?

texts:
The Forbes news article NASA's Hubble Telescope Snaps Best Images Yet Of Our First Interstellar Comet shows the image below (which I have since annotated) of comet 2I/Borisov (C/2019 Q4) and links to this series of Hubble images: https://archive.stsci.edu/proposal_search.php?mission=hst&id=16009

Looking through the previews of those images, they all seem to have streaks of similar length and direction, so I am assuming that the telescope is tracking the comet's motion in each exposure and the streaks are stars, suggesting that all the small dots are noise and not stars.

Is that right?
Is the fuzzy one an extended object?
What causes so many isolated pixels to be so much brighter than the background? Is this just the tail of a statistical distribution of shot noise, or are there other mechanisms that can produce single pixel noise many standard deviations above  the statistical shot noise distribution?


texts: Does Earth illuminate the unlit part of the Moon as

texts:
... today is August the 17th? I just saw the moon (4:30 CEST) and I have the strong impression that the unlit (by the Sun) part of the Moon is anyway "visible" as a very very dark grey portion.

It will be impossible to find it against the dark sky if not for the companion bright slice. What I see basically fills the round, just is smaller as compared to the bright part.

This latter effect it is explainable by the fact that bright lights appear to us bigger as compared to less luminous objects when put in direct comparison. This is matter of perception. As such my question is: I am also just perceiving the rest of the moon because I know is there or indeed I see something real?

To me, it is reasonable that Earth can illuminate the Moon, however I am not sure. Moreover, the "Earth phase" as seen by the Moon should be crucial here. If I am right, this might explain why sometimes I see the above phenomenon but more often I don't.


texts: Transforming Galactic Coordinates in Cartesian with Distance

texts:
Lets say I have galactic coordinates of an object, l and b.

I figured out that i can transform them to cartesian system using the formula:

$$
\begin{align}
X &= \cos(b)\cos(l)\\
Y &= \cos(b)\sin(l)\\
Z &= \sin(b)
\end{align}
$$

Giving me a vector on the celestial sphere. So far so good, now I want to know the exact position of the object. Using 1/parallax, I compute the distance. Therefore I multiply it
with my vector to find out the exact cartesian position of the object.

$$
\begin{align}
X &= \textstyle\frac{1}{p}  \cos(b)\cos(l)\\
Y &= \textstyle\frac{1}{p} \cos(b)\sin(l)\\
Z &= \textstyle\frac{1}{p} \sin(b)
\end{align}
$$
What I want to know is if this calculation is correct and if not, what do I have to change or reconsider? Thanks!


texts:
Adaptive optics generally apply slight changes to mirrors to account for atmospheric turbulence. These generally require either a mirror to be broken up into smaller mirrors, each with their own actuator, or for a single large mirror to be slightly deformed by many different actuators.

Is it possible to do much of this digitally?

Couldn't software slightly attenuate or amplify each pixel at each time-slice collected from a passive mirror? These changes would depend on the distortions measured by a guide star or laser (this digital system would still require a guiding light)? I suspect this would probably be less accurate than true adaptive optics, but it would also be far less expensive (as you wouldn't need so many actuators).

Has this approach been studied and dropped? Is it ridiculous and not even theoretically possible? I'm curious if anything has researched this idea.


texts: How many stars are there in a Globular Cluster of 10^5 solar masses?

texts:
I was wondering whether there is an easy way to approximate the number of stars in a Globular Cluster (GC) with 10^5 solar masses.

Can one, for instance just assume the GC is made of sun-like stars and therefore has 10^5 stars?
Or is this too simple?

Thank you for your answers!


texts: How to connect my phone to a motorized telescope via a cable?

texts: Some motorized telescopes (e.g. Celestron NexStar Series) have no WiFi by default. Can I connect my phone to them to be used as a hand control via a cable? The telescopes have AUX port and phones USB C ports. Even when they have hand control or even WiFi, connecting the phone by cable can be better. I've heard that some telescopes have issues with WiFi connection and that the hand control does not provide the same level of convenience as an Android app (e.g. SkyPortal).


texts: I think that the galaxy closest to the Earth, that is, to the Milky Way, is the nebula called Andromeda, which is 2.5 million light years away. Despite this great distance, M31 is visible, and very pretty. Are there any galaxies closer to the Milky Way than the Andromeda galaxy?


texts: How Does Earth Move In Relation To The Stars

texts:
In my simulation, I have stars scattered around Earth, all stars are the same distance from the center of the Earth and positioned based on their right ascension and declination like so:

I am wondering how the movement of Earth can be simulated based on the current time when running the simulation.

I am using the hyg v3 database which can be found here. I think that this dataset has the data necessary to find out the motion of stars, but I may be wrong.

Also, due to the structure of this simulation, with almost 120,000 stars, it is much more efficient to rotate/move the Earth rather than move all of the stars.

How can I show the stars visible over a location at a certain time by simulating the motion of the stars by rotating Earth?


texts: Why are rocky and small planets nearer to the Sun whereas big, gas giants are farther?

texts: Planets, including Mercury, Venus, Earth, and Mars are rocky planets and are small compared to gas giants. And all these planet are near to Sun. But mass is directly proportional to force. So why don't any planets lie between rocky planets. And if gas giants lie farther always they why does Pluto lie after all the gas giants?


texts: Can a telescope image be amplified to see more by adding distant lenses or mirrors?

texts: Could a lens or mirror be placed in orbit on the same flight path with other parts to make a telescope or on the horizon of the moon to see farther with the Hubble?


texts: Which data should I use to take the fast Fourier transform (FFT) to find time period for eclipsing binaries?

texts:
I am trying to use Kepler Data for Eclipsing Binaries to estimate time period, and then other parameters such as mass, eccentricity, semi-major axis, distance, etc. of the system. I want to write code in MATLAB which will use FFT. The available data has the following columns:

bjd(date) | phase | raw_flux | raw_err | corr_flux | corr_err | dtr_flux | dtr_err

Does the data require any cleaning?
If I want to take the FFT of the data to find time period, which data should I use?


texts: What book(s) should I read to get started learning about astronomy?

texts:
I'm an undergrad software engineer but I also love physics and math. I'm starting to get going in AI/Machine Learning techniques and I'm going to use NASA data-sets to conduct analysis. However, I am not really acquainted with the pieces that make up the universe. This is a GIANT problem for me! I could browse wikipedia for hours but I don't think that would fit well with my way of learning in a linear fashion. I do not have any background in astronomy, so:

I realize this is probably a bit hard to answer but what book(s) should I read first? What should I read after that? What about after that etc...?

I love learning and am more than prepared to take the time to work my way through them. Thank you in advance!


texts: Hypothetical planet jump: Will it perturb star systems?

texts:
In a more speculative corner of SE somebody was dreaming up a story of planets traveling between star systems through wormholes.
I was wondering how much the appearing/disappearing of a planet the size of Earth would perturb the orbits of the other bodies in a solar system like ours. After all, the bodies in the system all interact, and the current situation appears to me as the delicate result of an evolution during which bodies collided, were ejected or were nudged into specific (dynamic) "places" which are stable, for example because they resonate with or are far enough away from others.
How much havoc would the sudden appearance of an Earth-sized planet in the habitable zone wreak on this balance? Both long-term and short-term (on astronomical scales; short-term may be 100 years) disturbances are interesting. Would it directly perturb the orbits of nearby planets (in our case Earth and Mars) enough to be prohibiting? For example, even relatively minor changes of the orbit can have an impact on climate.
Could it lead to catastrophic events much later, for example because some Trojan bodies are pushed out of their Lagrange neighborhood and start marauding the Solar System, increasing the chance of catastrophic impact events on Earth or an inhabited Mars?


texts: What's a good step up from 7x50 binoculars?

texts:
I own a Nikon OceanPro 7x50 pair of binoculars, but I find that when I look at the moon, I would like more magnification.

My requirements are:

costs less than $400
weighs less than 10kg
the length is less than 1-2 feet
much higher maginification than 7x (I already have a pair of 7x binoculars, as I said)
at least an 100mm objective I (I already have a device with a 50mm objective)

What kind of device should I go for? A spotting scope? Binoculars? A telescope of some kind?

Ideally, I'd go for binoculars, so that I can see with both eyes, but I'm told binoculars don't come with objectives larger than 100mm, for my budget.

I could buy a spotting scope, like this, but Wikipedia says that spotting scopes are not designed for astronomy.

I live in an apartment, so I can't use a big, heavy telescope, like a 4-feet 20kg thing. I don't have room in my apartment for such a big telescope, and I can't carry it over the threshold into the balcony.

So, what kind of device should I consider, and what are the tradeoffs? What kind of astronomical objects will I be able to see?


texts: Why doesn't the black hole in the center of the Milky Way glow similarly to the famous M87 image?

texts:
The M87 image made some astronomers famous recently as the first image of a black hole.
In the Milky Way, it has been concluded that there must be a black hole due to the movement of stars near the center. But why is there no such light effect in our black hole, as there is in M87? Or is it there, but we cannot see it for some reason?


texts:
I'm recalling while taking a university astronomy class the professor brought a few of us up to the observatory, around sunset.  He showed us a star near zenith with the telescope.  After we left the observatory and were walking back to our dorms we thought we saw the same star, with the naked eye.  The sky still seemed somewhat bright.

Is that possible?


texts: Is there a type of star that emits relatively monochromatic visible light?

texts:
I was thinking about this question on Worldbuilding and the answers involving monochromatic ambient light, which got me wondering if there were any stars that emitted relative monochromatic light (a "very" narrow band, although I don't have a definition, but something narrow enough to produce the effect in that photo, atmospheric effects notwithstanding).

So my question is: Is there any type of star that emits an extremely narrow band of visible light (not necessarily exclusively visible, non-visible wavelengths can be included) that we have either observed or theorized to exist? "Narrow" as in, on par with a low pressure sodium-vapor lamp.

My Google searches are hindered by a lot of noise, mostly things like monochromatic star patterns, and a few hits regarding non-visible radiation.


texts: How can heavy elements exist when the universe is young compared to the lifetime of a star?

texts: Our sun is said to be incapable of producing heavier elements, and these are thought to originate from older stars dying and going supernova. If our sun is 4-5 billion years old, and has a lifetime of 10 billion years, how could an earlier sun have lived and died in the short time between the creation of the universe and the birth of our fine solar system?


texts: Does the cosmic microwave background recede at the speed of light?  Is it possible it recedes even faster because of the stretching of space?


texts: Wouldn't the rings of Saturn experience tidal effect?

texts: The "Lord of the Rings" has more than 60 moons, some of which are larger than Mercury. My question is, wouldn't the ring, which is mostly comprised of rocks, experience tidal effect whenever a moon is near?


texts: How exactly 'polar aligning' a Telescope with equatorial mount helps?

texts: What is true purpose of polar aligning? Last night, I looked at Mars. It was moving fast.  I still need to adjust RA axis to track fast-moving Mars. Why the telescope didn't  track it by itself?


texts: If the earth was not tilted, what would be the north star?

texts:
What is earth tipped toward? The simple answer is Polaris.  The axis (one end of it) points at Polaris, so that's what it's tipped to point at.  But when we saw "toward" what do we mean? When it is tipped "toward" one thing, what is it being tipped "away from"?

If earth were not tipped then the equator would lie in the same plane as earth's orbit.  Earth being tipped 23 degrees means it is tipped 23 degrees "away from" alignment with its orbit.  So a 0 degree tip would be "not tipped" and a 23 degree tip is tipped 23 degrees in some direction.  What direction is it tipped in?  Another way to ask this would be, what would it be tipped at if it was tipped a full 90 degrees in the general direction in which it is currently tipped 23 degrees?

Imagine a coordinate system with two axes in the plane of earth's orbit and one axis perpendicular to that plane.  Earth's orbit stays in the plane, and the angle that earth is tipped points to, let's say, negative X.  So the line through the north pole and south pole lies in the XY plane and the earth's orbit lies in the XZ plane.  I want to look at stars and be able to see those axes, so I want to know what stars they roughly point at.

A cleaner way to ask this might be, what would be the north star if earth wasn't tipped? What would be the south star? What if it was tipped 90 degrees? That gives four of the six stars I'm looking for. I'm not sure how to phrase the question for the other two.  I want to call these stars "untipped north", "untipped south", "winter", "summer", "spring", and "fall" because the direction of the stars would be the same as the direction earth is from the sun during the extremes of those seasons (northern hemisphere).

[EDIT]

The plane of earth's orbit is aligned with a celestial (infinitely distant) great circle called the ecliptic.  It inversely happens to also be the path through the sky that the sun takes over the course of a year.  There is a latitude/longitude coordinate system where the equator is the ecliptic called "ecliptic coordinates".  So the question is actually:

Using ecliptic latitude and longitude, what 6 stars are at:

0, 0
0, 90
0, 180
0, -90
90, 0
-90, 0


texts:
The Forbes article Ask Ethan: How Does Very-Long-Baseline Interferometry Allow Us To Image A Black Hole? includes an example of optical interferometry from the Large Binocular Telescope, shown below.

  The occultation of Jupiter's moon, Io, with its erupting volcanoes Loki and Pele, as occulted by Europa, which is invisible in this infrared image. The Large Binocular Telescope was able to do this owing to the technique of interferometry. Credit: LBTO

related: When will the next series of mutual eclipses of Jupiter's moons begin?

The LBTO consists of only two telescopes 8.4 m (330 inch) wide mirrors, with centres 14.4 m apart, so I would expect to see higher resolution in one direction than the other.

However in this GIF the resolution seems to be the same in both directions. While I see high spatial frequency ringing in the horizontal direction, I don't see any difference in resolution.

Since the event is so fast (probably minutes or seconds) there is no chance that the natural rotation of the baseline during the course of a night could have helped.

So I'd like to ask How does the Large Binocular Telescope resolve so well in both orthogonal directions simultaneously?

Open image in new window for full size:

Source

  The LBTI (green structure in the center of the frame) between the two 8.4m mirrors of the LBT.  High resolution image Credit: LBTO - Enrico Sacchetti


texts: Why don't we use amateur astronomers' telescopes to create a huge interferometer?

texts:
Some telescopes in space have been proposed to function as an interferometer.
Being placed several hundreds or thousands of kilometers apart they could actual image exoplanets directly. To my knowledge none of these space telescopes have yet been deployed.
So I was wondering why we don't connect all the several thousand amateur telescopes on Earth together to create a huge interferometer?
That sure would require some efforts (especially software to coordinate it all) but compared to sending telescopes into space it seems outright simple.
Are amateur telescopes simply not strong enough, even when linked together? What would be the best we could achieve?


texts: Fast moving bright star with eliptic course, instant stop

texts: Around 13 years ago I saw fast moving bright star with eliptic course which crossed half a sky in 1-2 sec and stopped instantly like it was a smashed like a bug, and didn't move for a while few minutes, then I went my way. I saw many falling stars and sattelits mouvements, planes on night, but never something similar.  Thank you in advance!


texts: Is there a consensus as to where terrrestrial planet atmospheres in our solar system came from?

texts: I tried to research this and I'm not sure if there is a consensus where the $CO_2$, $N_2$, etc. comes from on terrestrial planets like Venus, Earth and Mars. Possible sources would be accretion from the protoplanetary disk, outgassing (which I would think would require plate tectonics/ molten core) or reactions at the surface caused by stellar radiation. Also, reactions in the atmosphere can alter the composition. Would it be a combination of all of the above and, if so, do we know if one of the processes dominated over the others? Does anybody have a good link that discusses this?


texts: Why is Ganymede's aurora only visible in ultraviolet while Earth's is bright green and red?

texts:
GANYMEDE AURORAE

NASA's Hubble Space Telescope observed a pair of auroral belts encircling the Jovian moon Ganymede. The belts were observed in ultraviolet light by the Space Telescope Imaging Spectrograph and are colored blue in this illustration.

Why is Ganymede's aurora only visible in ultraviolet while Earth's is in the visible part of the spectrum?


texts: Why was the diaeresis removed from Pasiphae?

texts: The Gazetteer of Planetary Nomenclature notes that the spelling of Jupiter's moon Pasiphae was changed from Pasiphaë in July 2009, i.e. they removed the diaeresis. I've been unable to track down the rationale for this decision. Is anyone aware of why the spelling was changed?


texts: What would happen if a wormhole gets swallowed by a black hole?

texts:
Assume wormholes exist. One hole allows matter to pop out of another hole somewhere else in the universe.

If one end of the wormhole is swallowed by a black hole, then no information can get from that end to the other hole.

If the wormhole acts as some kind of gateway to another part of the universe, wouldn't that mean that the entire universe would be swallowed by the black hole? ie. the wormhole end is an entry portal to the entire universe, so if that is inside the black hole, wouldn't that mean the entire universe is then in the black hole?


texts: Is there any difference between M87's image and predictions?

texts:
Today, the EHT released (somewhat blurry as compared to simulations) images of the M87 black hole.
Did the image contain anything which might lead to a correction to General Relativity or were all the observations according to what had been predicted?


texts: Have non-periodic radio signals been analyzed for shannon entropy or put on a Zipf plot?

texts: If aliens did exist, they would probably compress their communications. And they might use a fault tolerant infrastructure like TCP/IP. In either scenario, real communications would not be periodic but closer to random noise. Has anyone performed a Zipf plot or calculated shannon entropy of non-periodic radio signals?


texts: How to convert quasar flux time series from observed frame to rest frame?

texts: I have flux time series of quasars in the observed frame and want to convert it in the rest or quasar frame. Any one who can help? Thanks


texts: How does the solar wind destroy the atmosphere of a planet lacking a magnetic field?

texts:
It is said that because Mars doesn't have a magnetic field its atmosphere disappeared, but how can solar wind cause that?

That the protons and electrons can changes molecules in an atmosphere I can understand, but how can solar wind cause molecules to escape from gravity?

Or is it that the atmosphere didn't escape in outer space but in contrary the molecules became too heavy, so they got solid and were pulled to the planet's surface?


texts: What is the last day a waning moon is visible in daytime (the afternoon)?

texts: From the fourth quarter onward, can we see the moon in the afternoon sky?


texts: Is it possible for planetary rings to be perpendicular (or near perpendicular) to the planet's orbit around the host star?


texts:
This arose from a comment on Worldbuilding.

We have data from four planets in the Solar System with rings, which doesn't make for a very good sample size. Observations of exoplanets could change that, but I expect that for now, any answer will simply be theoretical.

How thick can planetary rings be? I assume it would depend on the amount of material, but I don't know.


texts: How can ionized emission line flux decrease as a function of increasing metallicity or abundance?

texts:
The chemical evolution of galaxies is an important way to learn about their formation and stellar/gaseous constituents. Many galaxies show narrow emission lines at optical wavelengths (3500-9000 Angstroms) from ionized elements (e.g., [O II] at 3727 Angstroms) and recombination Balmer lines (e.g., H$\alpha$ at 6563 Angstroms). The origin of the ionized emission line flux can depend on the amount of the element present in the galaxy, and also on the nature of the ionizing source (i.e., the oft-mentioned "shape of the ionizing spectrum").

I have a few questions that I cannot seem to find clear answers to in textbooks, publications and review papers. My main reference for the following facts/questions is Kewley & Dopita 2002, ApJS, 142.

What is the difference between metallicity and abundance, and what impact would these two parameters have on the ionized emission line flux from a galaxy (assuming that the ionizing spectrum is kept constant)?
There are claims that some ionized lines (e.g., the auroral line [O III] at 4363 Angstroms) would become much weaker as the metallicity increases. Why? I would think that higher metallicity means more Oxygen and, assuming we keep the ionization spectrum fixed, more likely for more [O III] to exist. Should this not increase the [O III] 4363AA flux?
Similarly, some excitation line ratios (e.g., ([O II] 3727 + [O III] 5007)/H$\beta$, commonly known as $R_{23}$) are known to decrease with increasing abundance (measured via log(O/H)+12). Again, why? I would think if the metallicity or oxygen abundance is higher, then there is an increased chance of getting more [O II] and [O III] emission. Also, assuming that metallicity is higher, the amount of hydrogen should decrease, and this would decrease the $H\beta$ flux in the $R_{23}$ ratio above, thus increasing the ratio at high abundance.


texts: Sky-Watcher Dobsonian 16 Inch Synscan Go-To

texts: What is the best way to buy Dobsonian telescope 16‘? Where can I buy it new or used? What I will need to have in advance? Which negative sides it has? Please, share any information you have about this telescope. I will accept, any kind of advice, experience or whatever. Can it be used for earth objects and which kind of roof will it be needed?


texts: wavelength-based IFU data cubes VS velocity-based IFU data cubes

texts:
I have 2 simulated IFU data cubes of the same observation, let's say A and B. The 3rd dimension of cube A is in wavelengths. In order to create cube B I just rebinned cube A in log-wavelengths and then converted that to velocity (km/s). The rebinning to log-wavelength was necessary in order to convert the 3rd dimension of cube B in km/s. For an explanation see this:

Converting ångström spectral dimension to galaxy speed (km/s)

Now I want to apply instrumental broadening on both cubes. In order to do that, for each cube, I have to create a Gaussian profile with FWHM equal to the resolution of the instrument (or Line Spread Function, or LSF) and then convolve that with every spectrum of the cube. Of course the LSF of cube A will be in wavelengths while the LSF of cube B in km/s. Here is my question now:

Since all pixel shifts in cube A do not correspond to the same km/s value, one of the two LSFs won't be a perfect Gaussian, right? Shall I assume that would be the velocity LSF?
What is the correct way of creating the velocity LSF? Do I just create a wavelength LSF and then rebin it to km/s, the same way I rebinned cube A to cube B?
Does it worth the effort though? Will it make a difference? I have seen peer reviewed papers using velocity LSFs which are perfect Gaussians. Is this because the difference is negligible or my whole thinking/question is totally wrong?


texts: Is it possible to use Photolithography for telescope image sensor?

texts:
The current largest digital CCD camera is the Vera C. Rubin Observatory, which has a whopping 3.2 gigapixels with a pixel size of 10 micrometers. In contrast, Agfa 10E56 holographic film has a resolution of over 4,000 lines/mm—equivalent to a pixel size of 0.125 micrometers.
It makes me think. Will it be possible to use a chemical image sensor to achieve a nanometer pixel size?
Between 1983 and 2006 the Digitized Sky Survey(DSS2) produced 1.1-gigabyte images (23,040 x 23,040) of 15 micrometers corresponding to 1.0 arcseconds using Photographic-plates. One of their challenges was storage space. They were forced to use a lossy adaptive algorithm to shrink the output size, during which data was lost.
But since 2018, we can produce 7 nanometers chips via the Photolithography process.
Can the same technology be used for image sensors? What can we do in 2021?


texts: Can the Sun be used as a point source of light to achive better focus?

texts:
There's a limit to how well sunlight can be focused by a parabolic reflector or a lens because the Sun is not a point source. I wonder if the sunlight could be engineered to work as if it was (much more similar to) a point source.

If almost all of the Solar disc was covered by a coronograph, the light which gets through a small hole would be (almost) a point source. Is that correct?

If then an array of such covered reflectors or lenses concentrated their light to one and the same point, would a better focus be achieved than without covers?


texts: How does gravity interact with a photon?

texts: If photon is mass-less and gravity can interact only with matter, then how  does gravity alter the trajectory of light?


texts: Can a planet exist with the same mass as the earth, but have a different diameter?

texts: Can there be a planet with the same mass, and a similar composition, as the earth, but with a larger or smaller diameter, and how could that theoretically occur? I've tried to see if there is any research or sites that tackle this idea, but I, understandably, haven't found anything.


texts:
Although it is impossible to tell what is and isn't false color (aside from the adage that it probably is false).  It's inarguable that no picture of a gas giant shows any depth or significant transparency.  Of course this could just be because a single camera lacks depth perception, and since the planets are gas almost completely gas, there are no underlying boundaries to detect any transparency.

In particular what is the visibility inside the various conditions on Jupiter?  Could you see 1,000 km in every direction, or is it more like a constant fog or dust storm (with frozen ammonia instead of dust)?  What if you brought your own light source?


texts: Gas Planets composition and gravitational effects

texts: It has been postulated that Gas Giants have a solid core and an obvious gaseous atmosphere.  Question:  As the gas is a state of matter and gravity draws all matter inward, is the reason the gas is not drawn towards the core and condensed into a solid because the extent of the effect that the gravity has on the gas proportional to the mass of that gas.  Additionally, wouldn't this create a series of unique gaseous layers and wouldn't the depth of those unique layers be contingent on the layers composition.  Also, could the gravitational effect on the known layers be used to determine the composition of the core.


texts:
What are solar maximum and solar minimum?
Is there a relation between sun spots and solar maximum and minimum?
Thank you!


texts:
I have an ASCII file (.con) with the coordinates of vertices of different contours. This file is obtained from Matlab. Now after every set of contour ends there needs to be a blank row in the contour file. In place of this, I have a row with Nan Nan in it when Matlab writes the file. What I do is simply find and Replace the Nan with nothing. But when I load the contour file in ds9 it gives me weird contours. I'm also posting the result I expect (fig 1) and the result I'm getting (fig 2). The problem here that I've understood till now is that if I copy and paste every contour block of coordinates separately into the .con file the result is as expected but I want the whole process of writing the contours from Matlab into the file such that it gives me the expected result in ds9 to be automated (basically I don't want to copy and paste every block). Can somebody help with this?

This is what I expect:

Link for the .con file that gives the right result: https://drive.google.com/open?id=1tDMXtmywJnSjpi_CzCI5-F85oFpebnl4

Link for the .con file that gives the wrong result: https://drive.google.com/file/d/18rgN1UVtvcyfJSuh1mmbqOVQCna_aAUM/view


texts: what degree in the sky does the sun change color?

texts: I would like to know at what angle in the sky the sun changes from a white yellow to orange and red. I know that after sunset it's considered twilight until 18° beneath the horizon. So how many degrees above the horizon generally before the sun changes color?


texts: Enceladus; why use the words "geysers", "jets", and "plumes" interchangeably?

texts:
Background, Europa:

In an earlier question How can “Geysers” on Europa reach heights of 100km? I brought up the use of the word "geyser" in association with the reported confirming observations of water vapor plumes on Europa. Europa size and surface gravity are nearly as large as those of Earth's moon. With an escape velocity of about 2,025 m/s it was hard for me to understand how an actual geyser could exist within the ice, a geyser so powerful that it could be shooting liquid water from the surface at 2 kilometers per second.

Part of the problem here is that I still can't be absolutely sure there is a clear definition of what is or isn't an actual geyser on solar system bodies other than Earth. On Earth, while a geyser is usually powered by geothermal conversion of water into steam, it pushes additional liquid water up to and above the surface as liquid. I believe that other sources of pressure may still result in proper Earth geysers.

Most scientific references I found referred to the vapor as plumes or jets, the word geyser was used in more popular and general interested articles.

It seems one likely explanation is a venting of water vapor (gas) from a particularly shaped vent can accelerate the molecules to an average velocity that is several times the speed of sound by converting random thermal motion to directed vertical motion, as described in a U.T. Austin thesis by Jared James Berg. Simulations there showed that this jet phenomenon can result in some water vapor reaching a substantial fraction of escape velocity, and rise to 100 to 200km altitudes. The model also addresses the formation of solids as ice crystals within the plume.

About Enceladus:

I have just come across the paper How the Geysers, Tidal Stresses, and Thermal Emission Across the South Polar Terrain of Enceladus are Related, (Porco, DiNino and Nimmo (2014), Ast. J. 148 (3), 45), an in depth study of sources of water vapor from Enceladus. I saw that both geysers and jets were mentioned in the abstract, but at the end of the second paragraph in the introduction I ran into the very interesting sentence:

  (Throughout this paper, we use the terms “jets” and “geysers” interchangeably.)

The escape velocity on Enceladus is only about 240 m/s, nearly an order of magnitude lower than that on Europa, which means much less sub-surface pressure would be required to push liquid water high above the surface into near space. So I can understand that the term geyser might be more likely to be appropriate for these plumes than it would be for the plumes on higher surface gravity Europa.

Question:

I would know why the words "geysers" and "jets"/"plumes" would be used interchangeably. Is this because it hasn't been confirmed yet what they are, and the authors don't wish to show favoritism by choosing one or the other, or are they synonymous and can always be used interchangeably, or si there some other reason?

above x2: Figures 3a and 3b from Porco, DiNino and Nimmo (2014).


texts: What bright body is next to the Moon in these photos of the ISS transiting the lunar disk?

texts:
The question What are the chances that the ISS transits the moon as seen from your house (or headquarters)? links to the SpaceX Instagram-posted photos of the ISS transiting the Moon's disk as seen from SpaceX headquarters.

There's a bright object next to the Moon, but I'm not sure exactly when this happened. Instagram says "last weekend" which would appear to be the last weekend of July in 2018.

What bright object is this, next to the Moon? Planet? Star?

https://www.instagram.com/p/Bl8xjJFFrHM/


texts:
I want to take pictures of planets, well at least Mars, Jupiter and Saturn with some surface details.
I have a Canon EOS 7D Mark-II and a Tameron 150 - 600mm lens (very heavy), a 1.7X extender, and a... meh... half decent tripod that can take a peak in the sky directly overhead, with some neck discomfort. So far, I have been able to only capture a fuzzy blob of Mars with aura of orangish red light around it. But that is partly because the skies were not very clear, and some clouds quickly tried to hide the sky when they saw me taking a picture.  And it is Late October. So... Any suggestions, ideas,


texts: Does there exist any of open database or service with ALL discovered stars and their parameters?


texts:
I am interested in learning more about the (simplified) life cycle of gas planets which are not brown dwarfs (meaning less than 13 Jupiter masses). It obviously starts off with their creation within a protoplanetary disk. For Jupiter, things then happened quickly:

Jupiter formed in a geologic blink. Its rocky core coalesced less than a million years after the beginning of our solar system, scientists reported Monday in the Proceedings of the National Academy of Sciences. Within another 2 million or 3 million years, that core grew to 50 times the mass of Earth.

Source: Washington Post: Jupiter is oldest planet in solar system
But what happens afterwards? How long would a gas planet like Jupiter live (assuming that the central star of the system would not consume it)? What will happen to the radual density distribution over time? On which parameters (mass, radius, initial density, ...) does the evolution depend?
Related

Terminology question: gas giant vs gas planet
Will Jupiter eventually stop shrinking?


texts: How are the newly discovered Janus/Epimetheus rings different from the other rings of Saturn?

texts:
What are the newly discovered ring systems of Saturn, and the circumstances relating to the discovery? Is there something that makes them different from the old well-known rings, like their formation?

The ring in question is the Janus/Epimetheus ring system:


texts: Why has the orbit of Venus such a low eccentricity?

texts: The eccentricity of Venus is almost 14 times smaller than that of Mars, and 2.5 times smaller than that of Earth, and 30 times smaller than that of Mercury. Is it just a pure fluke? Or is there some “physical reasons”? I realize those are the ratios as of now, and that they change over time: will the evolution of those eccentricities dramatically change those ratios? If not, then the question of a “physical reason” emerges again.


texts: Currently proposed ways to distinguish between a really big planet and a really small star

texts:
The Next Big Future article Rogue Exoplanet 12.7 times bigger than Jupiter is 20 light years away

Astronomers using the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) have made the first radio-telescope detection of a planetary-mass object beyond our Solar System. They found a rogue planet 12.7 times the mass of Jupiter twenty light years from earth.
It is at the boundary of Jovian objects and Brown dwarf stars.
[...]
Last year, an independent team of scientists discovered that SIMP J01365663+0933473 was part of a very young group of stars. Its young age meant that it was in fact so much less massive that it could be a free-floating planet — only 12.7 times more massive than Jupiter, with a radius 1.22 times that of Jupiter. At 200 million years old and 20 light-years from Earth, the object has a surface temperature of about 825 degrees Celsius, or more than 1500 degrees Fahrenheit. By comparison, the Sun’s surface temperature is about 5,500 degrees Celsius.
The difference between a gas giant planet and a brown dwarf remains hotly debated among astronomers, but one rule of thumb that astronomers use is the mass below which deuterium fusion ceases, known as the “deuterium-burning limit”, around 13 Jupiter masses.

In the spirit of

Do astronomers generally agree that the distinction between comets and astroids is not so clear?
Is this object an asteroid or comet, and how can it produce so many tails?

and the relatively recent reclassification of Pluto as a dwarf planet and not a planet proper, I'd like to ask what are the ways that have are currently proposed and given consideration to distinguish between a really big planet and a really small star?
Is the "rule of thumb" proposed in the block quote the leading contender or are there other definitions that receive a similar amount of support


texts: If a white dwarf compresses to the limit of electron degeneracy, and a neutron star compresses to the limit of neutron degeneracy, what does a black hole compress to the limit of?


texts: Is there a name for the star/supernova hypothesized to have been a precursor to the Solar System?

texts:
I found an article suggesting a supernova injected debris which helped nudge the formation of the Solar System 4.5 billion years ago.

I couldn't find a name for this hypothetical star/supernova after some research. Is there one? What is it?


texts: Are there actual asteroids in the Oort cloud?

texts:
This answer to At what annual rate are new exoplanets being recognized? How does it compare to new asteroids? shows that the rate of new asteroid discovery is roughly two orders of magnitude higher than the rate of new exoplanet discovery, and once Vera C. Rubin comes online the disparity is likely to widen even further.
I quipped:

Thinking about the cross-over rate; at some point won't all the asteroids have been found?

there and then wondered if there is any distance limit to what is or isn't an asteroid, or are there new asteroids out there to discover all the way to the point where matter can no longer be gravitationally bound to the Sun because the pull of adjacent stars is just as strong.
Question: Are there asteroids per se in the Oort cloud, or for that matter, the Kuiper belt? Or are those basically considered comets? (trick question alert! see Do astronomers generally agree that the distinction between comets and asteroids is not so clear?)
Wikipedia sez the proposed Oort Cloud is full of icy planetesimals, and not being an astronomer I don't know how exactly to parse that.


texts: Which things can LIGO see that LISA can't, and vice-versa?

texts:
CNET's Astronomers discover two ferociously fast stars locked in a death spiral quotes Kevin Burdge, lead author on the new paper in Nature  General relativistic orbital decay in a seven-minute-orbital-period eclipsing binary system (no ArXiv?) and a Ph.D. candidate in physics at Caltech as saying:

  LIGO can see things that LISA can't, and LISA can see some things that LIGO can't, and there are a handful of things that might switch from being visible in LISA to LIGO over time"

Is there a simple way to understand which "things" fall into which categories?

LIGO is a ground-based gravitational wave observatory and is used together with Virgo, whereas LISA stands for Laser Interferometer Space Antenna and will be a GW sensing inteferometer based on a trio of satellites in an Earth-trailing heliocentric orbit.

GIF Source

  LISA spacecrafts orbitography and interferometer -yearly-periodic revolution in heliocentric orbit.


texts: How to find the radius of a satellite not knowing the mass

texts: If I know the semi-major axis, the density of the planet, and orbital period.. how can I find the radius of the satellite?  I can't think of any formula to use.


texts: Could there be additional Neptunian satellites between Larissa and Hippocamp?

texts:
Plotting the moons of Neptune in terms of semimajor axis and eccentricity using the values given on the Wikipedia page for the moons of Neptune, there appears to be a fairly large gap between the orbits of Larissa and the newly-named satellite Hippocamp.

Are there stable orbits within the gap, and are there likely to be additional moons there?


texts:
Hot Jupiters are close to their parent star - after all, that's why they're so hot. Their atmospheres are thought to be slowly blown away by the parent star, and they will eventually become chthonian planets.

Are there any other effects on Hot Jupiters because of their proximity to their parent star? Examples I'm looking for include issues with solar flares or coronal mas ejections, or even difficulties relating to the solar wind. I would also be tangentially interested in any effects the parent star would have on moon formation (or lack thereof).


texts: Transit Depth Color Versus Transmission Spectroscopy

texts:
One of the tools being developed for the study of exoplanets is called transmission spectroscopy - basically looking at how passing through the exoplanet's atmosphere changes the spectrum of the star. The problem, though, is that spectroscopy is hard, and requires a big telescope, probably space-based. This makes the process slow. It seems that it would be very useful if we would triage the data using broad-band photometry to more quickly limit the presence of compounds like water, $\mathrm{CO}_2$, and $\mathrm{O}_3$. Looking at a plot of Earth's atmospheric transmission, with a breakdown by chemical compound, it seems like looking at the color of the transit depths in and between the windows in the wavelength range from 1 to 20 microns, might provide a plausible alternative to quickly get information on the atmospheres of planets and brown dwarf stars.

For the exoplanets, specifically, the wavelengths blocked by an atmosphere make the planet, effectively, bigger at some wavelengths than at others, depending on the opacity at that wavelength, the thickness (density) of the atmosphere, and vertical extent of it.

The question is: is this practical? Specifically, with a 1 to 2 meter class space telescope that simultaneously images in 2 to 8 filters?


texts:
I live in northern hemisphere around 44th latitude. Starting from Winter Solstice, December 21, 2018 the duration of daylight is naturally increasing. What I observe, however, is that the duration increase is happening more in the evening rather than in the morning. In other words as days pass, the sunset seems to move forward much faster to a later hour than the first morning light moving backward to an earlier hour.

Is what I am observing a true phenomenon or is it merely a wrong observation? If so why is this happening in an uneven fashion?

EDIT

I found the sunrise/sunset times for my area. It in fact confirms the situation. In the first half of January sunrise moved back only 3 minutes while sunset moved forward 15 minutes with respect to civil time.


texts: Is the James Webb telescope going to orbit around the earth?

texts:
If it’s going to orbit around the earth, what kind of orbit would it be (what distance)?
Are there benefits to sending a telescope further away from the earth if it’s just to photograph galaxies and not objects that are in our solar system?


texts: I was wondering if the sun's radiation affects the gravity when it passes through space? Like can radiation strengthen or weaken the force of gravity?


texts: Scientifically important discoveries with the help of amateurs

texts:
I am looking for examples where amateur astronomers in these days contributed sigificantly to important scientifically discoveries.

I am aware of one example: Victor Buso from Argentina was lucky enough to observe a Supernova just when it formed, which has led to follow-up observations by large scientific observatories, and in the end resulted in a Nature article which has Buso as a co-author.

What might some other similar examples be?

below: "Astronomer Victor Buso poses in front of the telescope with which he witnessed the birth of the supernova 2016gkg. (Victor Buso)" From here, borrowed from Does this amateur observation hold a record even including robotic searches for supernovae?. Click/open for full size.


texts: Can you attach an old Vixen model to a celestron tripod?

texts: I would like to know if I can put an old Vixen VMC model (15 years old) onto a celestron 4se tripod. I am aware that some adaptations must be made, but I want to know if it's even possible or if it will miss alignments by far, or not work at all.


texts: Why were the Magellanic Clouds named that way?

texts: From the Wikipedia article, the Large and Small Magellanic Clouds were observed prior to Magellan's expedition, and the name only stuck long after Magellan's expedition (at first, they were called nubecula major and nubecula minor, or Large Cloud and Small Cloud). So why were they finally named after Magellan?


texts: Doesn't the energy conservation law imply, that the universe can't come to a Big Freeze?

texts: .. because, how would all the current matter and energy manifest then? Isn't everything coming to complete stop, minimal temperature, zero interaction state impossible? Wouldn't there still be some rare minor events occuring? And if so, given infinite amount of time couldn't these result in events resulting in some kind of chain-reaction?


texts:
The Sun loses about 5.5 million tonnes of mass every second, does this mean Sun's volume is also going down? If so can we tell by how much every year?
E:
"by how much" I mean can we tell if sun's radius is going down too


texts: What equipment do I need to measure the spectrum of a star?

texts:
For an amateur astronomer, is it possible to measure the spectrum of a star?

I can easily take a photo of a star just by putting a camera on one end of a telescope and over exposing it.

But I would like to measure the spectrum of a star. I imagine I could do some crude approximation by using different filters. But I would like the full spectrum to see what the composition of a star is.

How would I go about this?


texts: Black hole collision animation: What are these extrusions?

texts:
In this video uploaded by LIGO Lab Caltech, two inspiraling black holes are depicted. The video's description explains what is shown and can be summarized by:

  The colored surface is the space of our universe, as viewed from a hypothetical, flat, higher-dimensional universe, in which our own universe is embedded.

  ... the colors depict the rate at which time flows.

  [Space] is dragged into motion by the orbital movement of the black holes, and by their gravity and by their spins. This motion of space is depicted by silver arrows...

Just before things become quiescent, regions of space around the merging black holes extrude upward. (Actually, the extrusions seem to begin sometime before things get chaotic.) If "space" here is being represented like the rubber sheet analogy, what do these extrusions mean?

If gravitational forces create depressions in the sheet, then it seems to follow that those extrusions are anti-gravitational, which can't be right. Could they be regions where the equations modeling the black holes' interactions spit out nonsense?

(From video, timestamp 0:52)


texts:
Many books explain the difference between synodic and sideral periods. The synodic period of planets are tabulated for centuries and it is defined as the time interval between two consecutive repetition of phase, phase in this case meaning the terrestrial measure of its angular position and speed relative to the Sun.

My question is, how do astronomers measure the angular position of a planet relative to the Sun if the major observations are made during the night?


texts: Puzzling quotes from astronauts about earth size

texts:
I have no doubt that we have been to the moon. This question has nothing to do with a moon landing hoax.  But, there are two quotes from two different astronauts regarding the size of the earth as viewed from the moon that are puzzling to me. Both quotes talk about how small the Earth looked. Shouldn’t the Earth look very large when viewed from the moon

It suddenly struck me that that tiny pea, pretty and blue was the Earth. I put up my thumb, shut one eye and my thumb blotted out the planet Earth. I didn't feel like a giant. I felt very, very small. — Neil Armstong
As we got further and further away it [the Earth] diminished in size. Finally it shrank to the size of a marble, the most beautiful you can imagine. That beautiful, warm living object looked so fragile, so delicate that if you touched it with a finger it would crumble and fall apart. Seeing this has to change a man. — James Irwin

I know the term large is subjective, but still, the comments seem off. Please let me know what I’m missing.


texts: Do contact binaries such as Ultima Thule affect our current theories on how the Moon formed?

texts: While I was reading about Ultima Thule, I was wondering if the discovery of contact binary objects had any effect on our current theories of how the Moon formed?


texts: How can one determine the declination range of stars visible for a certain amount of time above a specific airmass at a certain location?

texts:
Let's assume you want to observe stars at Kitt Peak (32 degrees North) at an airmass above 2, for 2 hours (30 degrees of Earth rotation).

My attempt:

The angle between the zenith and celestial equator is 32 so the altitude to the celestial equator is 58 degrees and a star with a declination of 28 degrees south will cross the meridian at 30 degrees altitude at Kitt Peak (assuming my calculations are correct)... but will not be above the required airmass for 2 hours.

Here is where I'm stuck:

We know that one hour of time corresponds with 15 degrees. In this case, would I just subtract 15 degrees from the 28-degree star? i.e. 15 degrees to go up and 15 back = 2 hours. If this is correct the final southernmost declination star would be 13 degrees south. I'm not sure if my thinking is correct and I don't know how to check.


texts: If an asteroid was to collide with Earth, how much warning would we have?

texts:
I was talking to my 9-year-old son about the Florence asteroid that is passing close to the earth. He wanted to view youtube videos about that asteroids, and we saw the starting of this one https://www.youtube.com/watch?v=UzxMbkrNbcg and when I realized that the title video has 2036 as the year in which an asteroid crashes into the Earth. I explained that that was science fiction (like Back to the future) and only an animation

He asked me: "If an asteroid or similar object was to collide with the Earth,.... how many years or months before the crash occurs would the scientific community know?"

I suppose that the advance time depends on technology, so assume the current level of technology.


texts: Calculate Day of the Year for a given date

texts:
I am trying to make sense of the following formula that claims to calculate the day of the year given today's date represented by (day/month/year):

N1 = floor(275 * month / 9)
N2 = floor((month + 9) / 12)
N3 = (1 + floor((year - 4 * floor(year / 4) + 2) / 3))
N = N1 - (N2 * N3) + day - 30

Finally N supposedly refers to the day of the year.

What do the magic numbers 275,9 etc represent in the equation ?


texts:
In an article on Venus by JPL from 1996 I read

Due to the dense Venusian atmosphere, primary impact craters of less than a 3 kilometer (2 mile) diameter are nonexistent.

I am now wondering whether there is somewhere a comprehensive list of (other) directly observed strikes of incoming rocks  on Venus e.g. by atmospheric anomalities?
Side-question: What minimal size must an object entering Venus' have in order to reach the ground and/or leave an obserable trace on the surface?
References

Wikipedia List of craters on Venus would allow to compile a size distribution of all incoming objects larger than a certain size.
Meteor size and frequency distribution for Earth vs. moon or other planets?
Britannica on Meteor vs. meteoride vs. meteorite
How many meteorites have been found some place other than Earth? Where is an authoritative list maintained?


texts: Is it possible to tell if a certain extrasolar planet produces its own magnetic field?

texts: I know by observing the dimness of a star it is possible to calculate an exoplanet's distance from the star and its mass by how much the star wobbles. However, is there any way to accurately determine if said planet has any magnetic field through observation alone directly?


texts: how accurate are the predictions for coronal mass ejections-(location, timing and magnitude)?

texts:
I suppose this applies to all solar events.

I know centuries ago they knew the quantity of these things fluxuated in  solar cycles. Apart from that I know there has been advancement in solar weather forecasting but how accurate is it? Could, say ESA, have enough foreknowledge to launch a satellite or have one poised in solar orbit to 'go fishing' to investigate the emissions in someway? ?composition of particles- for example?


texts:
I am working on a little project where I want to simulate the solar system to teach myself working with python and OOP.

I want to calculate the orbits "real", from tangential velocities and gravitational accelerations. For this, in order to not only get circular orbits, but the real, eccentric orbits, I need kind of a snapshot of the solar system with positions and tangential velocities at those positions of the planets (and their moons, but I highly doubt that exists).

EDIT: I think something like the velocities at the Apoapsis should be enough to derive a reasonably right orbit from it, right?

I don't even know if something like that exists, or where to get it from. Does anybody happen to know something of this?


texts: How to find a place's latitude when you know a star's minimum and maximum altitude?

texts:
All we know that a star's altitude at its upper culmination is 38 degrees and 52 arcminutes and that its altitude at its lower culmination is 12 degrees and 24 arcminutes (so it never really sets at that place).
How can we, through this information, find that place's latitude?


texts:
We all know LIGO has detected gravitational wave(GW) directly. prl

The LIGO team argue they detected the merger of two solar mass black holes($36M\odot+29M\odot-3M\odot$) at z=0.09.

My questions are

It seems GW has Doppler shift too. There is a GW spectrum with emission or absorption lines like optical spectrum?

Figure.2 of the PRL paper seems to be very simple. Generally, people take their masses derived seriously? There may be other models which can give different masses. It is different from Taylor binary which was observed many years.

What does a 100HZ GW mean?  That is the frequency of the two arms shrink and extend?

What does the frequency stand for in the binary system?

The discovery can not be confirmed. DO you think the team win a Nobel prize only after other facilities like LISA find GW again?


texts: is there any theory or observational evidence that our universe is electrically neutral or not?

texts:
It seems our universe is neutral in large scale.

There is CP violationwiki problem about matter and dark matter.
Similarly is there any theory about whether our universe is electrically neutral?

Cosmic rays are not neutral. However the atomic nucleus and electrons are just separated. There should be no charge excess in total.

What is the largest scale that is severely non-neutral?


texts: Total rotational angular momentum estimates for Jupiter?

texts:
I wrote a quickie answer where I estimate the total rotational angular momentum of Jupiter as about 7E+38 kg m^2/s. However it bothers me because one of the sources gets this number assuming uniform density, and the other two cite 6.9E+38 kg m^2/s but it's not clear to me where this comes from.

Are there better estimates that use reasonable models for Jupiter's density distribution?

update: If not answerable due to uncertainties in density distribution and/or gradients in rotation rate (i.e. non rigid-body rotation) which I sort-of remember reading about recently, then an explanation why a better value isn't available would be an acceptable answer.


texts: How do we know that comets definitely mase and not just fluoresce?

texts:
Wikipedia's Astrophysical_maser#; comets mentions some anecdotes of notable masers associated with comets, and Maser emissions from comets begins:

  The 18-cm lines of the OH radical are the only well-documented masers in comets. They have been observed in more than 65 comets since 1973. The good knowledge of their excitation mechanisms and their linear regime allow us to estimate the water production rate and its variation with heliocentric distance from radio observations of OH in comets…

I'm guessing that OH around comets starts as water and is dissociated by UV light from the Sun. Either it left in a metastable excited state or it's excited (pumped) up to one by other photons from the Sun.

What is it about the 18-cm lines that allows one to conclude that this is masing; that there is microwave amplification by stimulated emission rather than just strong fluorescence?


texts: What is the condition for the number density of a gas to be ultra-relativistic or non-relativistic and degenerate or ideal

texts:
What is the condition for the number density ($n$) of a gas to be ultra-relativistic or non-relativistic and degenerate or ideal?

I found problems in this subject when I was reading about White-Dwarfs.

A book said $n \gg n_{QUR}$ or $n \ll n_{QUR}$ is the condition for being degenerate or ideal (classical), respectively. And $n_{QUR} = 8 \pi (kT/hc)^3$. Is this condition right? I know where $n_{QUR}$ comes from but I don't know what it really shows.

In that book it is said that $n \ll (mc/h)^3$ is the condition for being ultra-relativistic for a degenerate gas and $kT\gg mc^2$ is the condition for being ultra-relativistic for an ideal gas. Where do these equations come from? What is $mc/h$ ?

In another way, my question is mainly about this diagram and about how we can get the lines in this diagram.

Credit: The Physics of Stars, A.C.Phillips

$k$ Boltzmann constant
$h$ Planck constant
$c$ light speed
$n$ is number density, not the number of particles
$n_{QUR}$ is the quantum concentration number for ultra-relativistic particles


texts: Distance parallax and cosmological distance in universe in expansion

texts: I know that we cannot measure cosmological distances with parallax, but this is more a philosophical question. Am I correct in assuming that if we could measure distances using parallax (like stellar parlax used for nearby stars) the distance we would measure would correspond to the "angular diameter distance" ? https://en.wikipedia.org/wiki/Angular_diameter_distance


texts: Is the diameter of the observable universe a relative quantity?

texts: The diameter of my observable universe is 90 billion ly measured in proper distance. But isn't lenght a relative quantity in the theory of Relativity? Could an observer moving at a different velocity than me measure a different diameter for my observable Universe?


texts: When you travel fast around a black hole, do you experience high speed yourself?

texts: When an object in the space comes close to a massive body (like a black hole or a neutron star), we see from Earth that its speed significantly increases while it is close to the body. As we know, this is due to space distortion around massive objects known as gravity. But also, gravity causes time dilation, so an observer near a black hole will experience less time spans between same events than a far-away observer. So, the question is: if we travel on an asteroid and come close to a black hole, will we experience an increase in our speed, or the time dilation will compensate space distortion and we'll feel the same movement speed as before (i. e. no acceleration)?


texts: So I read that if an astronaut travels at the speed of light and goes to a nearby star system , when he or she returns, he or she will have aged 10 years but 1000 years will have gone by on Earth. Travel at the speed of light is not achievable ,so I was wondering if the spacecraft was traveling at an achievable speed of 10% the speed of light , how much will the astronaut have aged and how much time has elapsed here on Earth upon the return of the astronaut ?


texts: What is the difference between the Yarkovsky effect and YORP effect?

texts: As far as I understand, both the Yarkovsky effect and the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect exert forces upon bodies as a result of thermal photon radiation. How do both effects differ from one another?


texts: Hypothetically, could a civilization in a solar system near a supermassive blackhole use the blackhole as a huge gravity lens telescope?

texts:
I have heard of various proposals that use the massive gravity of stars to sort of focus light at and observe with a suitable telescope placed at such a distance, and I am assuming here that bigger the star in terms of mass, the more focusing and the better images the telescope sees
Assuming a hypothetical civilization, or perhaps humanity developed such a telescope, could they bring it feasibly close enough to a supermassive black hole at the center of say, our galaxy to observe insanely far away celestial bodies and bodies close by with incredible detail?
Typically, uptil what distance could we comfortably resolve details of say, the color and clouds of exoplanets, and view stars with this? A few hundred thousand light years atleast?


texts: How do they know how far out Farout is? (2018 VG18)

texts:
The Carnegie Science article Discovered: The Most-Distant Solar System Object Ever Observed begins:

Washington, DC— A team of astronomers has discovered the most-distant body ever observed in our Solar System.  It is the first known Solar System object that has been detected at a distance that is more than 100 times farther than Earth is from the Sun.
The new object was announced on Monday, December 17, 2018, by the International Astronomical Union’s Minor Planet Center and has been given the provisional designation 2018 VG18. The discovery was made by Carnegie’s Scott S. Sheppard, the University of Hawaii’s David Tholen, and Northern Arizona University’s Chad Trujillo.
2018 VG18, nicknamed “Farout” by the discovery team for its extremely distant location, is at about 120 astronomical units (AU), where 1 AU is defined as the distance between the Earth and the Sun.  The second-most-distant observed Solar System object is Eris, at about 96 AU.  Pluto is currently at about 34 AU, making 2018 VG18 more than three-and-a-half times more distant than the Solar System’s most-famous dwarf planet.
2018 VG18 was discovered as part of the team’s continuing search for extremely distant Solar System objects, including the suspected Planet X, which is sometimes also called Planet 9.  In October, the same group of researchers announced the discovery of another distant Solar System object, called 2015 TG387 and nicknamed “The Goblin,” because it was first seen near Halloween.  The Goblin was discovered at about 80 AU and has an orbit that is consistent with it being influenced by an unseen Super-Earth-sized Planet X on the Solar System’s very distant fringes.
The existence of a ninth major planet at the fringes of the Solar System was first proposed by this same research team in 2014 when they discovered 2012 VP113, nicknamed Biden, which is currently near 84 AU.

That's great! But the next bit confuses me.

The discovery images of 2018 VG18 were taken at the Japanese Subaru 8-meter telescope located atop Mauna Kea in Hawaii on November 10, 2018.
Once 2018 VG18 was found, it needed to be re-observed to confirm its very distant nature. (It takes multiple nights of observing to accurately determine an object’s distance.)  2018 VG18 was seen for the second time in early December at the Magellan telescope at Carnegie’s Las Campanas Observatory in Chile.  These recovery observations were performed by the team with the addition of graduate student Will Oldroyd of Northern Arizona University.  Over the next week, they monitored 2018 VG18 with the Magellan telescope to secure its path across the sky and obtain its basic physical properties such as brightness and color.
The Magellan observations confirmed that 2018 VG18 is around 120 AU, making it the first Solar System object observed beyond 100 AU.  Its brightness suggests that it is about 500 km in diameter, likely making it spherical in shape and a dwarf planet. It has a pinkish hue, a color generally associated with ice-rich objects.

Question(s):

How was the value of 120 AU actually determined?
or at least, how could it be determined if the explanation isn't readily available?
Is this measurement more of an orbit reconstruction, or a parallax measurement of current distance?

below: "Discovery images of 2018 VG18, nicknamed 'Farout,' from the Subaru Telescope on November 10, 2018. Farout moves between the two discovery images while the background stars and galaxies do not move over the one hour between images. Image is courtesy of Scott S. Sheppard and David Tholen." Source


texts: Could a pair of binary black holes form within a star?

texts:
After the discovery of gravitational waves by LIGO last week the team behind the Fermi gamma-ray telescope released a paper that showed a soft gamma ray burst was detected only 0.4s after the gravitational wave was detected at LIGO, and it was also located in the right area of the sky. This could be evidence of a gravitational wave source with a detected EM counterpart.

Ultimately though the Fermi group resist claiming a full discovery as no electromagnetic signature is expected form a binary black hole merger, but whats interesting is that I saw another paper today that claimed a GRB could be caused by a binary black hole merger inside a massive rapidly rotating star.

Is such a system realistic or even possible?


texts: What is the O[III]/H$\alpha$ line ratio for Active Galactic Nuclei?

texts:
I have the H$\alpha$ line flux of an AGN and I am trying to calculate the Bolometric luminosity using it. Bolometric luminosity can be calculated from O[III] fluxes by $L_{\rm Bol} = 3500 L_{\rm O[III]}$.
What is the $\rm O[III]/H\alpha$ line flux ratio for AGN?
I am new here, any help or redirection to an appropriate forum is immensely appreciated!


texts: How does the appearance of the analemma vary with latitude

texts:
How does the appearance of an analemma vary with latitude?

This what I take to be a definition of an analemma: If the position of the sun is plotted or photographed at noon clock time (say UTC) every day from a position facing due south (in the northern hemisphere) it forms a figure of eight shape as in the picture.

Photographs usually show it with the long axis at an angle. My question is, assuming that the angle is related to the latitude of the camera, how does this angle vary as the camera is moved from the equator to the pole, and why?


texts: Explanation of resonance stability and instability

texts:
I'm having some trouble grasping the idea of planetary resonances due to what seems to me like an ambiguity. It is known that the asteroid main belt is sculpted by the Kirkwood gaps, corresponding to orbits that are in mean motion resonance with the Jupiter. As such, the asteroids reaching such orbits get ejected (unstable) from such resonance orbits. Nevertheless, objects like Pluto are dynamically stable due to their resonance with Neptune. Mercury's spin-orbital resonance makes its elliptical orbit stable around the Sun.

Any ideas how to explain the two-fold nature of the outcomes of planetary resonances ? Any references to some literature / mathematical proof would be highly appreciated.

Thank you in advance,


texts: How many stars are visible to naked eyes around Equator area?

texts:
In the best sky conditions, we can see about 44,000 stars in the sky. But what if I am located around Equator area? How many stars I can see?

From what I know, the number of stars I can see should be far less ( I don't know why, though). Is it true? If yes, why?


texts: How will "modern equipment" allow the Royal Observatory to now avoid some of the effects of light pollution at Greenwich?

texts:
The article First Light: a new era for the Royal Observatory says (in part):

25 June 2018
The first modern, research-grade telescopes have just been installed at the Royal Observatory Greenwich, making the institution a working observatory once again after 60 years. Marek Kukula, Brendan Owens and Tom Kerss explain what this will mean for them, researchers, and the public.
"Modern equipment has allowed us to bypass a lot of the disadvantages that drove astronomy away from Greenwich in the middle of the twentieth century."

How exactly will "modern equipment" do this? A blocking filter for the bright yellow sodium line of some streetlights might have some effect, but with high pressure mercury lights and now broadband LED illumination, what other tricks do Greenwich astronomers have up their sleeves?

AMAT’s dual role is aptly matched to its name-sake. Annie Maunder explored the night sky and its phenomena as an astronomer, astrophotographer and science communicator in Greenwich during the 1890s and early 20th Century. She was hired as a human computer at the Royal Observatory in 1891, and though, as a government employee, she was required to give up her position when she married fellow Greenwich astronomer Edward Walter Maunder, she continued to participate in science, publishing books to inform and inspire others and returning to work at the Observatory during the First World War. She was also one of the first women to be elected as a Fellow of the Royal Astronomical Society in 1916. By the end of her life she had gained an international reputation, particularly for her work examining the Sun during eclipses for which she designed her own specialised cameras.


texts: Calculating RA/dec from JPL ephemeris data

texts:
I'm using JPL planetary ephemerides to calculate the position of planets. Using DE405 ephemeris (using a parser provided by Project Pluto with all tests passing). I am fetching the position of Mars with the observer set as the Earth (geocentric).

My first assumption is that the result is a set of geocentric equatorial coordinates in rectangular form (x, y, z). This may be incorrect.

My goal is to calculate RA/Dec. My second assumption is that I should use a formula converting from Cartesian to Spherical:

$r =\sqrt(x^2+y^2+z^2)$
$ra =\arctan(y/x)$
$dec = \pi/2 - arccos(z/r)$

Using Julian day 2457134 (21st Apr 2015, noon UTC), and cross-checking the result with NASA's Horizons service which apparently uses the same ephemeris (DE405) gets me almost there but my results are a little off:

x 1.721427968227801 y 1.5802812301744067 z 0.6897426775298559

RA 02h50m12.51s Dec +16°26'41.37"

Horizons:

RA 02h50m11.52s Dec +16°26'36.5"

I'm not sure what I'm missing. Do I have to factor distance of the planet due to the time it takes for light to reach us? Am I not using the correct time? Are my assumptions wrong?


texts: Can a Black Hole enter inside a Worm hole? If yes then what will a observer observe if it present at the another side of Worm hole?


texts:
Can someone explain what is happening with energy (e.g. light) and matter which was collected by a black hole?
I suppose that matter builds a black hole's mass and size, but what about light? Does it just disappear? But to generate light, the star consumed matter (mass).


texts: Clarifications about distances in cosmology

texts:
I would like to get clarifications about some usual notions of distances in cosmoogy.

First, the comoving distance is the current distance of objets whose light has been reached by us now, i.e regards to the cosmic time ? OR is it the same with redshift dependent ? if yes, this comoving distance is simply the angular diameter distance, isn't it ?

The angular diameter distance $D_A$ is the distance from observer to an object starting to emmit and such taht the light is reaching us now ?

The comoving distance transverse of an object is the size of object in 2D plane. From the following link : distances in cosmology

It is written :
5) Comoving distance (transverse)
The comoving distance between two events at the same redshift or distance but separated on the sky by some angle $\delta \theta$ is $D_{\mathrm{M}} \delta \theta$ and the transverse comoving distance $D_{\mathrm{M}}$ (so-denoted for a reason explained below) is simply related to the line-of-sight comoving distance $D_{\mathrm{C}}$ :
$$
D_{\mathrm{M}}=\left\{\begin{array}{ll}
D_{\mathrm{H}} \frac{1}{\sqrt{\Omega_{k}}} \sinh \left[\sqrt{\Omega_{k}} D_{\mathrm{C}} / D_{\mathrm{H}}\right] & \text { for } \Omega_{k}>0 \\
D_{\mathrm{C}} & \text { for } \Omega_{k}=0 \\
D_{\mathrm{H}} \frac{1}{\sqrt{\left|\Omega_{k}\right|}} \sin \left[\sqrt{\left|\Omega_{k}\right|} D_{\mathrm{C}} / D_{\mathrm{H}}\right] & \text { for } \Omega_{k}<0
\end{array}\right.
$$
with $D_H=\dfrac{c}{H_0}$

6 Angular diameter distance
The angular diameter distance $D_{\mathrm{A}}$ is defined as the ratio of an object's physical transverse size to its angular size (in radians). It is used to convert angular separations in telescope images into proper separations at the source. It is famous for not increasing indefinitely as $z \rightarrow \infty ;$ it turns over at $z \sim 1$ and thereafter more distant objects actually appear larger in angular size. Angular diameter distance is related to the transverse comoving distance by
$$
D_{\mathrm{A}}=\frac{D_{\mathrm{M}}}{1+z}
$$

Question 1) in the part 1) they define the comoving distance by :
$$D_{\mathrm{C}}=D_{\mathrm{H}} \int_{0}^{z} \frac{d z^{\prime}}{E\left(z^{\prime}\right)}$$
But this is not the physical distance. Indeed, we need to multiply $D_{\mathrm{C}}$ by $R(t=0)=R_0$ to get the physical diatance between us and the object, don't we ?
Question 2) I don't understand in the text above why they say that "but separated on the sky by some angle $\delta\theta$ is $D_M\delta\theta$ and the transverse comoving distance $D_M$ is simply related to the line-of-sight comoving distance $D_{\mathrm{C}}$.
Should it be rather the current size of object is equal to $D_C\,\delta\theta$, i.e the comoving distance multiplied by the $\delta\theta$ ?, like we do in trigonometry for computing a portion $\delta\theta$ of circle perimeter.
Question 3) Concerning the angular diameter distance, why could we rather take the ratio cosmological_horizon/1+z instead of the ratio $\dfrac{D_{\mathrm{M}}}{1+z}$ ? Indeed, angular diameter distance is the distance at z given between the observer and the object emitting.
I make confusions between comoving distance transverse and angular diameter distance as well as comoving distance (coordinates or physical distance with the factor $R(t)$ ?)
I hope to have been clear. Any help is welcome.


texts: What is Copernicus referring to when he states that "circles have poles different [from the earth's]"

texts:
In chapter 4 of "The Revolutions of the Heavenly Spheres" where Copernicus maintains that the objects in our solar system have circular orbits or orbits compounded of several circles, the Edward Rosen translation of the book states this

  It stands to reason, therefore, that their uniform motions appear nonuniform to us.
  The cause may be either that their circles have poles different [from the earth's] or
  that the earth is not at the center of the circles on which they revolve.

To add context: Copernicus believes that features of orbit that are inconsistent to circular motion such as retrograde motion, apogee, perigee, fast/slow movement, etc... Don't actually occur in the orbits of the Sun, Moon, and the planets. Stating that the actual causes of the inconsistencies could either be that "their circles have poles different [from the earth's.]" Or that these objects don't revolve around the Earth.

I'm mainly concerned with what Copernicus is referring to when he states that the orbits of these objects "have poles different [from the earth's,]" and how (if true) it could produce these inconsistencies in orbits.


texts:
In Star Wars, the planet Naboo has a plasma core, instead of a molten one.

Can a planet really have a plasma core?


texts: Does the galaxy look like a Web or a Tree?

texts:
Suppose we have a distance cutoff $r_0$.
Let's say that any two stars in the galaxy with a relative distance below $r_0$ have a path between them.
Would the path-connected Milky Way look like a Web in which every star is potentially connected to all of its neighbors or would we form a type of branched Tree structure with junctions and bottlenecks?
I understand that for a high enough $r_0$, it will always look like a web. But my doubt is whether the standard-deviation of interstellar distances in the Milky way is such that there exists a value of $r_0$ for which the topology is unequivocally a Tree.


texts:
A little over half a year ago, I think it was mid March 2015, I glanced out of my window at about 4 am, to see what appeared to be a very bright total lunar eclipse, as in, the moon was full and bright orange-red. As I watched, it disappeared before my eyes, looking as if someone slid a paper down in front of it. The moon was not full that night (and by extension that means that there couldn't have been an eclipse that night) and the next few nights, the moon was on the other side of the sky at 4 am.

The room I was in was dark, so it couldn't have been a reflection on the window of something inside the house. It might have been a reflection on the window of something outside the house, but that's not very likely, because it was very bright and opaque (something would have to be really bright to look so bright and opaque when reflected off a window, and I can't think of anything that would have been so bright. Plus, I probably would have noticed something something so bright), it didn't move when I moved my head around, and it looked exactly like the moon – it had the same dark and light spots. I would have taken a picture of it, it would be a lot easier to understand what I mean if you would see it, but it disappeared really fast.

It might have been a reflection on the clouds. I don't recall if there were clouds in the sky, I wasn't paying that much attention to the sky around it, because while it was happening I just thought it was an eclipse. But if it was a reflection on the clouds, there's still the question of what was being reflected.

I asked someone who knows a lot about a bunch of topics, but is not good at explaining what he means, what it could have been, and he said something about solar flares, I didn't really get what he was saying. I would ask him again, but I don't know him personally and I don't know how to contact him, and in any case, I don't think he would explain it any better the second time.

I searched everywhere I could possibly think of (on the web, on Q&A sites, on SE) for a cause for this, but couldn't find anything like it.

What I'm asking is this: Could it have been a reflection on the clouds, and if so, what could it have been of? Could it have been caused by a solar flare or something like it, and if so, how? Is there an astronomical event that occurred around that time that could have caused something like that, and if so, what and how?

This question was put on hold as unclear what you're asking, so if anyone has any suggestions that can make the question more clear, I would love to hear it. I wish I could be more precise about when it happened, or what exactly it looked like, but I didn't write it down or note it in any way at the time, so this is pretty much all I remember.


texts: Are accretion disks the only way stellar planetary systems form?

texts:
The accepted theory for the formation of our Solar System is from an accretion disk.  A molecular cloud is thought to have collapsed under its own gravity and formed a disk.  Most of the matter in the cloud formed the Sun, and the remaining matter was either ejected or formed the planets and other bodies (e.g. moons, asteroids, comets).
Question:  Now that we've discovered many extra-solar planets, are there theories differing from the accretion disk theory for the formation of planetary stellar systems?


texts: What experiment does this "Tracing the History of Starlight" video try to explain?

texts:
The YouTube video Tracing the History of Starlight with NASA's Fermi Mission describes observation by the Fermi gamma ray telescope  of EBL (extragalactic background light) gamma rays from 2,000 Blazars. It seems to suggest that they used an attenuation measurement of the gamma rays to determine... I'm not exactly sure exactly what was determined by the investigation discussed in the video.

The attenuation is due to reverse pair-production, two photons in, and an electron-positron pair out. I am not sure if they measured 511 keV gamma rays from subsequent annihilation of the positrons, or attenuation of the primary gammas, and I am also not sure if the experiment was used to measure the primary gamma ray rates, or the total density of UV-Vis-IR light photons with which the gamma rays annihilated.

Question: Is there a simple but physically clear way to explain all of this; the experiment the video is supposed to be explaining?

How do they differentiate attenuation due to revere pair production from attenuation by say Compton scattering considering dust and gas?


texts:
Why does the Parker Solar Probe slow down as the distance from the Sun increases?

Image credit: Wikipedia user Phoenix777, CC BY-SA 4.0


texts: Can the difference between a star and a galaxy which are point sources be detected?

texts:
Obviously a star would be a point source. A galaxy should be an irregular blob if close, but if it is far away then it would seem that a galaxy too would be just a point source.

Given that the star and galaxy were both only detectable as point sources can astronomers tell them apart with redshift? By some other method?

A followup question...

What percentage of galaxies if our universe can we only detect as point sources?


texts: How did pre-Earth's mass and orbit change at the Moon formation event?

texts:
The popular, but still disputable, Moon formation theory is that a pre-Earth was hit by a Mars sized planet. The mass ratio of Earth:Mars:Moon is roughly 100:10:1.

Does this mean that pre-Earth was roughly 9 Moon masses (~10%) lighter
than today's Earth? Or could much mass have been ejected from the
Earth-Moon system?
Did the impact substantially and immediately change Earth's orbit,
such as its average distance from the Sun?
Is there a better planet name for "pre-Earth", like there is for
Theia? It was after all another planet than Earth is.


texts: What is the relevance of the Tropics and the Polar Circles? Do they Exist on other planets?

texts:
The middle line is the equator, the second and fourth line are the tropics, the first and fifth are the polar circles.

What is the relevance of these bands on Earth? Do these exist on other planets? What purpose do they serve?


texts: Moving from mathematics to astronomy/astrophysics

texts:
I'm currently studying for a PhD in pure mathematics (the topic is the cohomology of differential graded categories, just to give you an idea of how pure I mean), although the first year of my undergraduate degree was in mathematical physics before I transferred to mathematics, so I have a little (but unfortunately not that much) background in physics as well.
I'm seriously thinking about whether I want to pursue pure mathematics as a career, and since astronomy and astrophysics were, one could say, my first love when it comes to science-y type things, I've been wondering if it would be possible for me to switch fields, and what would be necessary to do so. Although I'm aware that astronomy/astrophysics is a wide area and could mean lots of different things in terms of a profession.
To give you a little more background, my current level of astrophysics knowledge is propped up mostly by my short time studying physics at university and a continuing interest and reading various non-technical books and low level textbooks of various topics. The topics I've found most interesting are things like supernovae, and the formations of stars and star systems, although I haven't studied the mathematics or technical details of these things in any serious depth. I should say that I split off from applied mathematics some time ago, so topics such as PDEs and statistical analysis are not areas I'm adept in (although I have no doubt that I would have relatively little trouble learning them at this stage). I've also spent a short amount of time (~ 1 year) working as a software developer, so I have some proficiency with programming. And actually I thoroughly enjoyed my time as a developer, and I know that software development/programming is an important aspect of work and research in this field for analysing images, data sets, creating simulations and that sort of thing. So something of that sort is of great interest to me, especially if it could directly involve mathematics. So perhaps working with simulations would be a good option for me?
Is a change of this kind possible for me? And if so what would you recommend I do to make it happen?
Apologies if this question is off-topic by the way. Please do let me know if there are more appropriate places to find this information.


texts:
I am trying to wrap my head around Gamow energy and it’s various terms.
To put it simply I understand higher Gamow energy reduces the chance of penetration relating to the Coulomb barrier.
For the width/window would it be fair to say that a higher value indicates a bigger window so therefore more chance of fusion occurring?
To be clear i am not asking for equations or help with any specific problem sets in nuclear fusion but I hoped some more knowledgeable people than myself could guide me on some simple understanding of the process.
I hope that makes sense?
Thank you in advance.